linux-kernel - Re: [PATCH] iommu/amd: Modify set_dte_entry() to use 128-bit cmpxchg operation

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Message-ID: <e937e26f-038a-6d01-76a9-76c86760ca4a@gmail.com>
Date: Mon, 19 Aug 2024 20:15:19 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Cc: joro@...tes.org, robin.murphy@....com, vasant.hegde@....com,
 jgg@...dia.com, kevin.tian@...el.com, jon.grimm@....com,
 santosh.shukla@....com, pandoh@...gle.com, kumaranand@...gle.com
Subject: Re: [PATCH] iommu/amd: Modify set_dte_entry() to use 128-bit cmpxchg
 operation



On 19. 08. 24 18:18, Suravee Suthikulpanit wrote:
> The current implementation does not follow the 128-bit write
> requirement to update DTE as specified in the AMD I/O Virtualization
> Techonology (IOMMU) Specification.
> 
> In addition, the function is used to program several DTE fields
> (e.g. stage1 table, stage2 table, domain id, and etc.), which is
> difficult to keep track with current implementation.
> 
> Therefore, introduce new a new dte256_t data type and a helper function
> update_dte_256(), which uses two try_cmpxchg128 operations to update
> 256-bit DTE.
> 
> Also, separate logic for setting up the GCR3 Table Root Pointer, GIOV, GV,
> GLX, and GuestPagingMode into another helper function set_dte_gcr3_table().
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>

[...]

> +static void update_dte256(struct amd_iommu *iommu, u16 devid, struct dte256 *new)
> +{
> +	struct dev_table_entry *dev_table = get_dev_table(iommu);
> +	struct dte256 *ptr = (struct dte256 *)&dev_table[devid];
> +	struct dte256 old = {
> +		.qw_lo.data = ptr->qw_lo.data,
> +		.qw_hi.data = ptr->qw_hi.data,
> +	};
> +
> +	/* Update qw_lo */
> +	if (!try_cmpxchg128(&ptr->qw_lo.data, &old.qw_lo.data, new->qw_lo.data))
> +		goto err_out;
> +
> +	/* Update qw_hi */
> +	if (!try_cmpxchg128(&ptr->qw_hi.data, &old.qw_hi.data, new->qw_hi.data)) {
> +		/* Restore qw_lo */
> +		try_cmpxchg128(&ptr->qw_lo.data, &new->qw_lo.data, old.qw_lo.data);

You should use plain cmpxchg128() when the result is unused.

Uros.